G7® and Color Calibration Explained In-Depth

Between 2004 and 2006, the IDEALLiance GRACoL (General Requirements for Applications in Commercial offset Lithography) committee worked diligently to define a standard appearance for commercial printing and proofing at the request of agencies and publishers. The first approach was to follow the industry-standard ISO 12647, but the result was too much visual inconsistency between different presses and printing methods.

The committee then tried Don Hutcheson’s Proof-to-Press method (now known as “G7”), which focused on gray scale calibration, rather than separate CMYK scales. Several carefully-controlled press runs carried out between 2004 and 2006 led to the development of standard Neutral Print Density Curves (NPDC) and gray balance formula, which form the foundation for G7 calibration. The GRACoL committee found that when traditional ISO TVI curves were replaced with G7 NPDC curves, there was a major improvement in print similarity between presses and proofing systems. An important by-product of the GRACoL project was the G7 specification which defined a “universal gray scale appearance” that can be applied to all printing systems, regardless of ink color, substrate or technology, and a simple calibration method for achieving it.

TONALITY AND GRAY BALANCE:

Most printing systems prior to G7 were calibrated using dot gain, also known as Tonal Value Increase (TVI). The GRACoL committee observed that TVI calibration alone of cyan, magenta, and yellow inks did not control the gray balance or the tonality of a print. The example swatches below have the same TVI, but different tonality. This is because TVI is a ratio of reflectance, not a direct indicator of appearance. This is evidenced in the following swatches of CMY gray where the TVI of each color is 16%:

This issue was addressed with the introduction of the G7 NPDC FanGraph (See Exhibit A for an example of the G7 NPDC fangraph), which provides target curves for G7 calibration from the average curves observed during the GRACoL Project print runs. Most people forego the FanGraph method and use software such as Curve3 to create calibration curves, however, the FanGraph method demonstrates the mechanics of G7 calibration.

One of the most important benefits of the G7 calibration process is higher accuracy and more consistency in lighter tones. All G7 prints have the same highlight tonality regardless of the maximum density of the print, which yields more consistent prints because the human eye is much more sensitive to tonality changes in the highlights. This can be seen on the highlight section of the target calibration curve on the FanGraph:

The FanGraph “fans out” as the dot percentage increases towards the shadow areas because G7’s Neutral Print Density Curves are adaptive. This means the calibration curves automatically adapt to devices with low maximum densities (with less shadow contrast) as well as devices with higher maximum densities. The CMY curves and K curve have separate NPDC target curves. The maximum density of each initial measurement dictates the target G7 curve: the target curve is drawn between the two curves that most closely match the test print’s maximum density. In the following example the maximum density for the CMY curve is 1.18; therefore, the target curve is drawn between the curves that intersect on 1.13 and 1.20:

The calibration curve allows higher or lower contrast in the shadows while still maintaining consistent tonality in lighter tones where the human eye is more sensitive to changes. The key to making an image look its best on systems with widely differing natural contrast is to allow shadow contrast to vary, while maintaining the same highlight contrast between systems.

Another advantage of the G7 methodology is that the gray balance formula takes into account the color of the print material. As a result, when printing on a device that is G7 calibrated, images printed on different colored substrates will appear visually neutral when compared to one another.

This works because G7 defines the colorimetric value of grays with CIE L*a*b values at specific points along a CMY curve. For the gray balance calculation, the existing CMY curve from the NPDC graph becomes the Cyan curve and the M and Y are recalculated based on the gray balance formula. The gray balance formula takes into account the a* and b* values of the substrate.

For example, the unprinted substrate an a* value of 3 and a b* value of -5 on the 37.5% point on the curve, the following equation is used:

This equation takes into account the relative whiteness of the paper so that when building the CMY curves the grays are balanced. The a* and b* values are used with the gray finder at the specific percentage (37.5 in example above) in order to provide any necessary changes to the M and Y curves so that there is gray balance between the C, M, Y curve and the substrate.

The cyan curve remains the same as the old CMY curve, while the M and Y curves are recalculated based on the colorimetric difference recorded on the grayfinder block. With these values, new G7 curves can be plotted into the RIP.

One of the reasons for G7’s runaway success is the fact that G7 does not apply to just one print device or process. Its common gray balance and neutrality rules can be used on almost color imaging method no matter what inks, substrate, mechanics or screening technologies it uses. As long as separate CMYK (cyan, magenta, yellow, and black) or CMY (cyan, magenta and yellow) percentage-based calibration tables are available in the RIP the print can be calibrated to G7 specifications.

G7 CERTIFICATION:

G7 Expert: A G7 expert is trained in the field of color management, process and quality control for proofing and printing equipment.

G7 Professionals: A G7 Professional is generally an in-house quality/technical professional.

G7 Master Qualification: While the previous three certifications apply only to individuals, the G7 Master Qualification applies only to facilities, equipment or systems.

WHAT G7 ISN'T:

While G7 has been a great asset to the printing industry, it can’t do everything. Here are a few of the common misconceptions:

G7 is not a color management system: G7 does not replace ICC color management because its one-dimensional calibration tables are not a substitute for the multi-dimensional variables that the International Color Consortium (ICC) color management solves. As a result, most proofing systems will need additional color management along with their G7 calibration. Typically, the easiest way to go about this is to create an ICC profile of the proofing system in its G7-calibrated state.

G7 is not its own official ISO standard: G7 is, however, a central part of four new ANSI standard color spaces, and G7 innovations like the near neutral calibration concept have had a sizable impact on the way modern printing standards are being applied.

Appendix A

Appendix B

As an IDEAlliance certified master printer, Signazon implements G7 in all aspects of print. To learn more about G7, contact us at 1.800.518.1217.